The invention relates to gas injection systems, particularly of methane, for internal combustion engines, for the type comprising:                a plurality of electromagnetically controlled injectors, associated to the various cylinders in the engine,        a distributing manifold or rail communicating with said injectors,        a feeding reservoir of the distributing manifold where pressurised gas is accumulated,        a pressure regulating valve interposed in the connection between the reservoir and said distribution manifold,        an electronic control unit set up to control the injectors and to control the opening time to meter the amount of gas injected in each cylinder according to the operating conditions of the engine and,        an on-off valve arranged on the output of the reservoir.        
The on-off solenoid valve complies with standards in force and ensures the possibility of hermetically closing the reservoir in any emergency condition, for example in the case of an accident entailing leakage of gas into the tubes located downstream to the reservoir.
The on-off valves used to date in systems of the type specified above use a deformable metallic material valve member, typically made of synthetic rubber, with consequent problems of tightness when cold. The use of plastic material valve members is consequently not advantageous, because there is the risk of the material forming the valve member being cut or sheared consequent to the pressure with which it is pressed against the corresponding valve seat, which again causes problems of tightness.